This invention relates to a liquid crystal display device, more particularly, to an in-plane switching mode liquid crystal display device having a plurality of electrodes formed on its the upper and lower substrate.
Recently, the TN-LCDs have been extensively used in devices such as the portable personal computer, measurement apparatus and the like. However, TN-LCDs, while having a number of advantages, also have the disadvantages of a narrow viewing angle and low response time.
To solve this problem, a multi-domain LCD such as a two-domain TN LCD(TDTN LCD) and a domain divided TN LCD(DDTN LCD), and a TN LCD including an optical compensation film have been introduced. In this LCD, however, a contrast ratio is decreased and a color shift is generated according to a viewing direction.
Further for the purpose of a wide viewing angle, an in-plane switching mode LCD is also proposed.
FIG. 1 is a plan view of TFT array substrate of the conventional in-plane switching mode LCD. As shown in figure, data electrodes 5 and common electrodes 6, which are applying electric field to the liquid crystal layer, are arranged in the pixel region defined by a gate bus line 1 and a data bus line 2 crossing each other. At the cross of the gate bus line I and the data bus line 2, the thin film transistor on which a gate electrode 3 and source/drain electrodes 4 are respectively connected to the gate bus line 1 and the data bus line 2 is positioned. When the voltage is applied to the data electrodes 5 and the common electrodes 6, the electric field is created in plane of the surface of the substrate. In the in-plane switching mode LCD, in other word, the liquid crystal molecules are rotated in plane of the surface of the substrate. Thus, the angular dependence problem caused by the refractive anisotropy of the liquid crystal molecule is lowered, namely a wide viewing angle could be achieved.
FIG. 2a is a sectional view taken along line A-A' of the FIG. 1, as shown in figures, a gate insulator 15 is formed on the common electrode 6 formed on a first substrate 10. The data bus line 2 and the data electrode 5 are formed on the gate insulator 15. A passivation layer 16 is formed on the data bus line 2 and the data electrode 5, and a first alignment layer 21a is formed on the passivation layer 16. Further a black matrix 8 is formed on a second substrate 11 to prevent the light leakage onto the gate bus line 1, the data bus line 2, and TFT. A color filter layer 17 is formed on the second substrate 11. A second alignment layer 21b is formed on the color filter layer 17. A liquid crystal layer 19 is formed between the first substrate 10 and the second substrate 11. Further a polarizer 25 is attached on the first substrate 10, and an analyzer 26 is attached on the second substrate 11 in which optical axes of the polarizer 25 and analyzer 26 are orthogonal perpendicular to each other.
Accordingly as shown in FIG. 2a, when an electric field is not generated between the data electrode 5 and the common electrode 6, the light is not transmitted in the liquid crystal layer 19 because liquid crystal molecules are parallel to polarization axes of the polarizer 25.
While, when an electric field is generated, as shown in FIG. 2b, between the data electrode 5 and the common electrode 6, the light is transmitted in the liquid crystal layer 19 because the long axis directions of the liquid crystal molecules depart from polarization axes of the polarizer 25.
In this in-plane switching mode LCD, however, since the data electrode 5 and common electrode 6 are formed on the first substrate 10, it is impossible to obtain an uniform electric field in the whole liquid crystal layer 19.
Therefore, it is necessary to provide a large driving voltage in order to sufficiently drive the liquid crystal molecules 20.
Further the contrast ratio is decreased according to viewing direction because the electric field is not parallel around the data electrode 5 and the common electrode 6 by an inclined alignment of the liquid crystal molecule according to the electric field.